Magnetic alloy

ABSTRACT

A cobalt base alloy is described having highly desirable mechanical and magnetic properties which alloy is suited for use at temperatures of up to 750* C. The cobalt base alloy contains nickel, iron, aluminum, tantalum, zirconium and boron, and optionally may contain titanium, niobium and beryllium.

llnited States Patent [72] Inventor Klaus Detert Niederrod, Germany [21]Appl. No. 820,655

[22] Filed Apr. 30, 1969 [45] Patented Nov. 2,1971

[73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

Continuation-impart of application Ser. No. 584,368, Oct. 5, 1966, nowabandoned.

[54] MAGNETIC ALLOY 10 Claims, No Drawings [52] US. Cl 75/170, 148/31.55[51] Int. Cl ..C22c 19/00,

[50] Field of Search 148/31.55, 32.5, 158,162;75/170, 171

[56] References Cited UNITED STATES PATENTS 2,829,048 4/1958 Cochardt eta1. 148/158 X 2,981,620 4/1961 Brown et a1 [48/325 X 3,331,715 7/1967Bulinoetal l48/1S8X 3,415,643 12/1968 Freche et al. 148/3 X PrimaryExaminer-Charles N. Lovell Att0rneysF. Shapoe and R. T. Randig ABSTRACT:A cobalt base alloy is described having highly desirable mechanical andmagnetic properties which alloy is suited for use at temperatures of upto 750C. The cobalt base alloy contains nickel, iron, aluminum,tantalum, zirconium and boron, and optionally may contain titanium,niobium and beryllium.

MAGNETIC ALLOY This is a continuation-in-part of application Ser. No.584,368 filed Oct. 5, 1966 now abandoned.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of section 305 of theNational Aeronautics and Space Act of i958, public law 85-568 (72 STAT.435; 42 U.S.C. 2457). This invention is directed to a cobalt-base alloyand to members made from the alloy which, after appropriate heattreatment, exhibit highly desirable magnetic and mechanicalcharacteristics.

in the hostile, nearvacuum environment of space, where the heatgenerated by the components of spacecraft and space stations, isdifficult to dissipate, such components will be expected to function atelevated temperatures in excess of 600 C. In particular, the rotors forelectric motors and generators aboard a spacecraft or space stationwould be required to function even though the component or ambienttemperature reached such high levels.

The majority of creep resistant alloys which have been developed forapplications at temperatures in excess of 600 C. are nonmagnetic, eventhough such alloys are generally either iron-nickelor cobalt-base, Theferritic creep resistant alloys, which do possess ferromagneticproperties, have a temperature ceiling for operation of about 500 C.Even after relalively short service at temperatures above 500 C. theseferritic alloys suffer a great loss in their mechanical strength. Themost recent advances in the art indicate that the temperature ceilingfor operation of ferritic alloys can be increased to some extent, butthat the maximum operating temperature for even these newest alloys willapparently not exceed 600 C.

in U.S. Pat. No. 2,829,048 issued Apr. 1, 1958, there is disclosed acobalt-base high. damping alloy which is characterized by substantialcreep resistance at temperatures up to 650 C. and is particularlysuitable for use in members such as turbine blades. This prior art alloycomprises essentially, by weight, from 65 percent to 88 percent cobalt,1 percent to 2 percent titanium, from 0.] percent to 1.5 percentaluminum, the total of aluminum and titaniumbeing at least 1.5 percent,carbon below 0.1 percent and preferably not exceeding 0.05 percent, andthe balance, at least 8 percent, and preferably between 16 percent and25 percent, being nickel with incidental impurities not exceeding 1percent. This alloy is ferromagnetic to a substantial degree attemperatures up to 650 C.

It is the object of this invention to provide alloy members having highcreep resistance combined with ferromagnetic properties at temperaturesup to 750C.

It is another object of this invention to provide a precipitationhardened cobalt-base alloy member containing predetermined amounts ofnickel, iron, aluminum, tantalum, zirconium and optionally boron, thealloy exhibiting high creep resistance and good ferromagnetic propertiesat temperatures up to 750 C.

it is a further object of this invention to provide a precipitationhardened cobalt-base alloy member containing predetermined amounts ofnickel, iron, aluminum, tantalum with one or both of the elementstitanium and niobium, zirconium and optionally boron, the alloy havinghigh creep resistance and good ferromagnetic properties at temperaturesin excess of 600 C.

Other objects of the invention will, in part, be obvious and, in part,will appear hereinafter.

The alloy composition disclosed provides a high creep resistance andgood ferromagnetic properties including high permeability at relativelyhigh ambient temperatures. This alloy is cobalt'base with a facecentered cubic lattice, which is precipitation hardened by a y(gammaprime) phase of the type Ni Al. The nickel atoms in this phase arein part replaced by cobalt atoms and the aluminum atoms are replaced inpart by tantalum atoms, or by a suitable combination of tantalum andtitanium, tantalum and niobium. or tantalum with both titanium andniobium.

More particularly, the invention is directed to a precipitationhardenable cobalt-base alloy characterized by good creep resistance andferromagnetic properties at temperatures up to 750 C., the alloycomprising 10 percent to 20 percent by weight of nickel, 5 percent to l0percent by weight of iron, 1.2 percent to 2 percent by weight ofaluminum, 3 percent to 6 percent by weight of tantalum, the total ofaluminum and tantalum not exceeding 7 atom percent, and the ratio ofaluminum to tantalum as expressed in atom percent being between 1:1 and2:], from 0.1 percent to 0.3 percent by weight of zirconium, fromoptional to 0.002 percent by weight of boron and the balance cobaltexcept for small amounts of incidental impurities. The above-describedalloy may also contain an effective amount of beryllium up to 0.2percent by weight as a strengthener.

A preferred alloy composition comprises from l2 percent to l6 percent byweight of nickel, 5 percent to 6 percent. by weight of iron, 1.2 percentto 1.5 percent by weight of aluminum, 5 percent to 6 percent by weightof tantalum, from 0.1 percent to 0.3 percent by weight of zirconium,from 0.0005 percent to 0.002 percent by weight of boron and the balancecobalt except for small amounts of incidental impurities.

As indicated above, the total amount of the hardening elements tantalumand aluminum is limited in the alloys of the invention to 7 atom percentof the alloy. Titanium and niobium may be used in conjunction withtantalum and aluminum up to a maximum of 2 percent by weight of eachwith beneficial result, however, the total amount of these four elementsin the alloy should not exceed 7 atom percent with the atom ratio ofaluminum to the sum of the other elements being between lzl and 2:1. inany case, tantalum is present in the alloys of this invention in anamount of from 2.5 percent to 4 percent by weight when used withtitanium and/or niobium.

One aspect of the invention involves the improvement obtained by thesubstitution of tantalum in whole or in part for the titanium employedin alloys of this type which rely for their improved properties uponprecipitation of the 7' phase. As has been indicated, the tantalumreplaces at least some of the aluminum in the y precipitate. By thisaddition of tantalum an improvement of strength is achieved in the alloycomparable in magnitude to that obtained with a titanium addition,without, however, as great asacrifice in magnetic properties.

Another aspect of the invention involves measures to avoid or suppressthe occurrence of any discontinuous or cellular grain boundaryprecipitate. A cellular precipitate can be developed in precipitationhardened cobalt-base alloys and has a remarkable effect in increasingthe damping capacity of these alloys. Methods for obtaining suchcellular precipitates in cobalt-base alloys, and the improved dampingcapacity thereby achieved, has been pointed out in copending U.S. Pat.No. 3,33l,715 which issued July 18, I967 in the names of Bulina andBrown.

in addition to its beneficial effect upon damping properties. a cellularprecipitate also has the effect of improving the short time tensileproperties. However, the long time creep properties are adverselyaffected by such precipitate, and this is seriously detrimental in thecontemplated applications of this alloy. Though a small amount of suchcellular precipitate for example, less than 3 percent, by volume can betolerated in the alloy, the development of a greater volume ofdiscontinuous precipitate at the grain boundary during the agingprocedure or during service must be avoided.

For the above reason, the total amount of additions which tend topromote the formation of the cellular precipitate must be carefullycontrolled. Since aluminum and tantalum play a large role in theformation and in the promotion of the cellular precipitate structure, ithas been found that the total amount (by atom percent) of aluminum plustantalum, or aluminum plus tantalum, titanium and niobium when theselatter elements are present, should not exceed 7 percent, and the ratioof aluminum atoms to tantalum (or tantalum plus titanium and/or niobium)atoms should be approximately 2:1, but not TABLE 1 [Properties of basealloys] Measured at room temp.

Aged 1 hr. at 750 0., Aged 1 hr. Aged 100 hr. measured at 600 C. at 700C. at 700 C. I\lXIagn.

0111. .S. Alloy Composition, wt. percent c.g.s. y Y I60 VH N ie 1BV-100-5, l e-0.27 Zr-14.6 Ni-1A4 Al-1.66 Ti-5.20 Ta 01 1-B-V-4 00-5,Fer-0.26, Zr-14.s,'Ni-1.25,Lil-5.19, Pa 100 31 g 1-B-V 00-5, ire-0.27,Zr-9.85, Ni-l.60, Al-5.3, Ta 105 2. 7 320 0' a l-BV-6 00-5, Fe0.24,Zr-15.0, Ni-1.19,.Al-1.66, 'Ii-3.1, Ta 100 2. 0 338 14' 2 1-13-s-2.C0-5.2,Fe-15.2,Ni-4.98,Ta-1.36,AH).12,Zr-0.065,Be-0.0 102 2.0 330 18 71-B-S-1. 00 5.3, rte-15.3, Nt-4.98, Ta-1.28,A1-0.21,Zr-0.002, B- 104 3.5331 107 Alloy N 000.3, Fe-0.2, 2:03, Si-0.3, Mg-23.4, Ni-OA, Al-1.7, Ti100* 5. 5 302 1710 Commercial heat treatment. Alloy N is a commercialdamping alloy.

less than 1:1. Since small amounts of zirconium tend to suppress theformation of cellular precipitate, the presence of up to 0.5 percent andpreferably about 0.1 percent to about 0.3 percent zirconium is highlydesirable. Boron may optionally be present in an amount of up to about0.002 percent and is effective for suppressing the formation of cellularprecipitate. When boron is present the combined zirconium and boronshould not exceed about 0.5 percent.

Since cellular precipitate is the product of prolonged aging incobalt-base alloys, short time aging of l to 5 hours at temperaturesbetween 700 C. and 800 C. is preferred to produce precipitationhardening. A two-stage aging treatment may also be employed; the firststage being the above outlined short time aging, and the second stagebeing an aging treatment at temperatures below 700 C. for several hours.The two-stage aging treatment provides a means for obtaining highershort time tensile strength while avoiding development of cellularprecipitate.

Still another novel aspect of the invention relates to the suppressionof the cobalt-rich hexagonal e (epsilon) phase during cooling of thealloy to room temperature. The existence of such an e phase in thematrix increases the coercive force of the alloy member to a substantialdegree, and therefore would decrease the permeability at roomtemperature. To accomplish the suppression of this undesirable 6 phaseat least 5 percent by weight of iron is added.

The alloys of the invention can be produced by melting the ingredientsby nonconsumable arc melting or in a vacuum induction furnace. Forexample, following the nonconsumable arc melting procedure employing atungsten electrode, the alloy was cast into ingots under an argonatmosphere at a pressure of 200 Torr. The ingots were hot rolled atl,l00 C. and then cold rolled to sheets.

in the vacuum induction melting procedure the alloy ingredients weremelted and cast into pound ingots using an MgO crucible in a vacuuminduction furnace operating at 10 kilocycles per second. In this lastinstance the basic alloy components iron, nickel, and cobalt were meltedin the crucible under a vacuum of 2 10 Torr. The alloying elements werethen added as master alloys under argon at a pressure of 280 Torr. Themolten alloy composition was cast at the same low pressure of argon inan iron mold which yielded a rod-shaped ingot 5 cm. in diameter. In eachcase this ingot was remelted by using it as a consumable electrode in avacuum arc furnace at a vacuum of l 104 Torr. An ingot of about 7.5 cm.diameter was obtained. After removing the surface layer of the in got,the ingot was hot forged at l,l50 C. to a bar of X50 mm. cross section.From each of these bars a piece was cutoff, hot rolled at l,050 C. andthen cold rolled to sheet.

The final heat treatment of the cold rolled sheet consists of ahomogenization anneal of l,000 C. or higher in a preheated furnace withargon or helium atmospheres, followed by moderately rapid cooling (20 toC. per minute). Aging treatments to produce precipitation hardening arecarried out in a bath of alkali and alkaline chlorides between 600 C.and 800 C.

Magnetic saturation measurements were taken on small samples (2.5millimeters in diameter and length). These sampics are placed in the gapofa magnet with a field gradient of 1,000 oersteds per centimeter and amean field of 1 1,500 oersteds. The measured values are determined withan accuracy of 1 percent, as magnetic moment per gram (0) in c.g.s.units, which can be converted into magnetic saturation B, expressed ingauss by B,=41-r 8Xo'. The density 6 is approximately 8.8 grams percubic centimeter. The values of magnetic moment given below, whenmultiplied by 1 l0, represent the saturation in gauss with a possibleerror of5 percent.

The coercive force was measured on specimens 35 mm. long, 8 mm. wide and1.5 mm. thick with an accuracy of2 percent by a precision type magneticfield probe and a magnetizing coil. In the following table of thecomposition, certain physical and magnetic properties of the alloys ofthe invention are set forth along with similar data for a damping alloyof the prior art.

The compositions listed in the first column of the above table wereobtained by chemical analysis ofthe ingot. it will be noted that thefirst four alloys are free of deliberate additions of boron while alloysl-B-S-2 and l-B-S'l have specific boron additions. The next threecolumns contain properties measured at 600 C., as magnetic moment/g.yield stress at 2 percent offset on flat samples and the coercive force.The last four columns contain the values of hardness and coercive forcemeasured at room temperature during isochronal aging at 700 C. for 1hour and hours respectively. These values are compared to the commercialdamping alloy. It is clear that both the boron containing and boron-freealloys demonstrate a substantial improvement in yield strength, magneticsaturation and coercive force. The listed alloys of the invention do nothave detectable discontinuous cellular precipitate, while the commercialdamping alloy has such cellular precipitates to the extent of about 15percent of the observed cross sectional area.

Additional DC and AC magnetic properties were determined for ring-shapedspecimens of alloy lBSl and are compared with the properties of acommercial damping alloy in table ll below.

TABLE II [Magnetic properties of alloys at elevated temperatures- Ringspecimens: Du 75 mm., Di=62.5, 0.625 mm. thick] DO Properties 'Iest t mpB (100 he.) B (250 01!.)1 Alloy C. kg. kg. H r 0 00.

AC Properties Test temp., Blip, Frequency Alloy 0. kg. (cycl/sec.)Pc/lh- Pmb.

lBSl 650 6 400 7. 74 8. 06 Alloy N 595 6 400 41. 3 77 The testtemperature in the above table is somewhat higher for the alloy of thisinvention than for the commercial alloy;

i.e., the test conditions for lBSl alloy are more rigorous than forAlloy N. Despite this, the DC induction is identical for the twomaterials and the coercive force is better for the alloy of theinvention by a factor of almost 30. As far the AC properties, the corelosses are nearly 6 times greater for the commercial alloy and theexciting volt amperes per pound is more than 8 times higher for thecommercial alloy than for alloy 11381.

In the manufacture of this alloy the procedures followed should besimilar to those employed in producing super alloys. in order to obtainthe magnetic properties as specified, segregations and inhomogeneitiesare avoided or eliminated during the processing. Vacuum arc melting ofthe alloy with chill casting is a preferred ingot making process, withlong soaking periods at temperatures around l,200 C. or higher appliedbefore of after hot rolling.

Annealing of the alloy at the final gauge should be done at temperaturesin excess of l,050 C. for a reasonable time with air cooling. The mostsuitable aging temperatures are between 700 and 800 C. as indicatedabove.

The alloys of this invention are primarily intended for use in the rotorand stator structures of electrodynamic machines such as motors andgenerators. While it is common for such structures to be assembled froma plurality of laminations, and the alloys of the invention may-be usedin this form; it is also contemplated that part or all of the rotor ofstator structures percent by weight of iron, from 1.2 percent to 2percent by weight of aluminum and from 2.5 percent to 6 percent byweight of tantalum, the sum of aluminum plus tantalum not exceeding 7atom percent of the alloy, and the ratio of aluminum to tantalumexpressed in atomic percent being within the range between 1:1 and 2:1,up to about 0.5 percent by weight of zirconium, optional to 0.002percent by weight of boron and the balance essentially cobalt except forsmall amounts of incidental impurities.

2. The alloy of claim 1 containing an effective amount of beryllium ofup to 0.2 percent by weight.

3. The alloy of claim 1 wherein the tantalum is present within the rangebetween 2.5 percent and 4 percent by weight and at least one metalselected from the group consisting of titanium, niobium and mixturesthereof, said titanium when present not exceeding 2 percent by weightand said niobium when present not exceeding 2 percent by weight, the sumof aluminum plus tantalum plus titanium plus niobium not exceeding 7atom percent of the alloy, with the ratio of aluminum atoms to tantalumplus titanium plus niobium atoms being within the range between 1 :1 and2:1.

4. The alloy of claim 3 containing an effective amount of beryllium ofup to 0.2 percent by weight.

5. A precipitation hardenable cobalt base alloy consisting essentiallyof from 12 percent to 16 percent by weight of nickel, from 5 percent to6 percent by weight of iron, from 1.2 percent to 1.5 percent by weightof aluminum, from 5 percent to 6 percent by weight of tantalum, from 0.1percent to 0.3 percent by weight of zirconium, from 0.0005 percent to0.002 percent by weight of boron and the balance essentially cobaltexcept for incidental impurities.

6. The alloy of claim 5 containing an effective amount of beryllium ofup to 0.2 percent by weight.

7. A dynamoelectric machine rotor or stator formed of an alloy havingthe composition of claim 1.

8. The alloy of claim 5 in which there is present at least one metalselected from the grou consisting of titanium, niobium and mixturesthereof in whlc the sum of the aluminum plus tantalum plus titanium plusniobium does not exceed 7 atomic percent and in which the ratio ofaluminum atoms to tantalum plus titanium plus niobium atoms is withinthe range between 1:1 and 2:11

9. The alloy of claim 8 containing an effective amount of beryllium ofup to 0.2 percent weight.

10. The alloy having the composition ofclaim 1 in which the alloyingcomponents are selected to produce a microstructure in the precipitationhardened condition characterized by having less than 3 percent by volumeofcellular precipitate.

2. The alloy of claim 1 containing an effective amount of beryllium ofup to 0.2 percent by weight.
 3. The alloy of claim 1 wherein thetantalum is present within the range between 2.5 percent and 4 percentby weight and at least one metal selected from the group consisting oftitanium, niobium and mixtures thereof, said titanium when present notexceeding 2 percent by weight and said niobium when present notexceeding 2 percent by weight, the sum of aluminum plus tantalum plustitanium plus niobium not exceeding 7 atom percent of the alloy, withthe ratio of aluminum atoms to tantalum plus titanium plus niobium atomsbeing within the range between 1:1 and 2:1.
 4. The alloy of claim 3containing an effective amount of beryllium of up to 0.2 percent byweight.
 5. A precipitation hardenable cobalt base alloy consistingessentially of from 12 percent to 16 percent by weight of nickel, from 5percent to 6 percent by weight of iron, from 1.2 percent to 1.5 percentby weight of aluminum, from 5 percent to 6 percent by weight oftantalum, from 0.1 percent to 0.3 percent by weight of zirconium, from0.0005 percent to 0.002 percent by weight of boron and the balanceessentially cobalt except for incidental impurities.
 6. The alloy ofclaim 5 containing an effective amount of beryllium of up to 0.2 percentbY weight.
 7. A dynamoelectric machine rotor or stator formed of analloy having the composition of claim
 1. 8. The alloy of claim 5 inwhich there is present at least one metal selected from the groupconsisting of titanium, niobium and mixtures thereof in which the sum ofthe aluminum plus tantalum plus titanium plus niobium does not exceed 7atomic percent and in which the ratio of aluminum atoms to tantalum plustitanium plus niobium atoms is within the range between 1:1 and 2:1. 9.The alloy of claim 8 containing an effective amount of beryllium of upto 0.2 percent weight.
 10. The alloy having the composition of claim 1in which the alloying components are selected to produce amicrostructure in the precipitation hardened condition characterized byhaving less than 3 percent by volume of cellular precipitate.